Method of making ceramic composite articles by inverse shape replication of an expendable pattern

ABSTRACT

There is disclosed a method of producing a self-supporting ceramic composite body having therein at least one cavity which inversely replicates the geometry of an expendable pattern. The method includes the steps of surrounding the expendable pattern with a filler material to thereby form a filled cavity within the filler material. The expendable pattern is chemically or physically removed from the filler material and a quantity of a parent metal is put into the cavity. The parent metal is heated to a temperature above its melting point and an oxidation reaction process begins whereby the oxidation reaction product infiltrates and embeds the surrounding filler material. Excess filler material and/or excess parent metal are removed, thus resulting in a self-supporting ceramic composite body having a cavity in the shape of the expendable pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Commonly Owned CopendingU.S. patent application Ser. No. 907,919, filed Sept. 16, 1986, and nowabandoned in the names of Andrew W. Urquhart and E. Allen LaRoche, Jr.,entitled "Method of Making Ceramic Composite Articles by Inverse ShapeReplication of an Expendable Pattern".

FIELD OF THE INVENTION

The present invention broadly relates to methods of making ceramiccomposite bodies having one or more shaped cavities therein. Inparticular, the invention relates to methods of making ceramic compositebodies comprising a polycrystalline ceramic matrix infiltrating a bed offiller and having at least one cavity of selected geometry formed byinversely replicating the shape of an expendable pattern.

BACKGROUND AND DESCRIPTION OF COMMONLY OWNED PATENT APPLICATIONS

The subject matter of this application is related to that of CommonlyOwned U.S. Pat. No. 4,828,785, which issued on May 9, 1989, from U.S.patent application Ser. No. 823,542, filed Jan. 27, 1986, and entitled"Inverse Shape Replication Method of Making Ceramic Composite Articlesand Articles Obtained Thereby" in the names of Marc S. Newkirk et al.This patent discloses a novel oxidation method for producing aself-supporting ceramic composite body having therein at least onecavity which inversely replicates the geometry or shape of the parentmetal precursor as the positive pattern. Thus, the resulting compositeproduct has a cavity of a predetermined geometry. This patent isdiscussed in greater detail below. Composites generally utilizing thesame oxidation phenomenon, but having no defined or predeterminedconfiguration, are disclosed in Commonly Owned U.S. Pat. No. 4,851,375,which issued on July 25, 1989, from U.S. patent application Ser. No.819,397, filed Jan. 17, 1986, which was a continuation-in-part of Ser.No. 697,876, filed Feb. 4, 1985, and now abandoned both in the names ofMarc S. Newkirk et al and entitled "Composite Ceramic Articles andMethods of Making Same." This patent discloses a novel method forproducing a self-supporting ceramic composite by growing an oxidationreaction product from a parent metal into a permeable mass of filler.

The method of growing a ceramic product by this oxidation reaction isdisclosed generally in Commonly Owned U.S. Pat. No. 4,713,360, whichissued on Dec. 15, 1987, from U.S. patent application Ser. No. 818,943,filed Jan. 15, 1986, as a continuation-in-part of Ser. No. 776,964,filed Sept. 17, 1985, and now abandoned was which a continuation-in-partof Ser. No. 705,787, filed Feb. 26, 1985, and now abandoned which was acontinuation-in-part of Ser. No. 591,392, filed Mar. 16, 1984, and nowabandoned all in the names of Marc S. Newkirk et al and entitled "NovelCeramic Materials and Methods of Making the Same." This patent disclosesthe method of producing self-supporting ceramic bodies grown as theoxidation reaction product of a parent metal precursor which may beenhanced by the use of an alloyed dopant. Molten parent metal is reactedwith a vapor-phase oxidant to form the oxidation reaction product. Inthe appropriate temperature range, molten metal is progressively drawnthrough the oxidation reaction product and into contact with the oxidantthereby continuing to form additional oxidation reaction product anddeveloping the ceramic body. The method was improved upon by the use ofexternal dopants applied to the surface of the precursor parent metal asdisclosed in Commonly Owned U.S. Pat. No. 4,853,352, which issued onAug. 1, 1989, from U.S. patent application Ser. No. 220,935, which wascontinuation of U.S. patent application Ser. No. 822,999, filed Jan. 27,1986, and now abandoned which was a continuation-in-part of Ser. No.776,965, filed Sept. 17, 1985, and now abandoned which was acontinuation-in-part of Ser. No. 747,788, filed Jun. 25, 1985, and nowabandoned which was a continuation-in-part of Ser. No. 632,636, filedJul. 20, 1984, and now abandoned all in the names of Marc S. Newkirk etal and entitled "Methods of Making Self-Supporting Ceramic Materials".

The entire disclosures of each of the foregoing Commonly Owned Patentsand Patent Applications, which are assigned to the same owner, areexpressly incorporated hereby by reference.

There is an increased interest in substituting ceramics for metalsbecause, with respect to certain properties, ceramics are superior tometals. There are, however, several known limitations or difficulties inmaking this substitution such as scaling versatility, capability toproduce complex shapes, satisfying the properties required for the enduse application, and costs. The above-described Commonly Owned Patentsand Patent Applications overcome these limitations or difficulties andprovide novel methods for reliably producing ceramic materials,including composites.

The invention described in Commonly Owned U.S. Pat. No. 4,828,785,(identified above) eliminates the difficulties in formation of ceramicbodies having shapes with complicated internal cavities and especiallyshapes with re-entrant cavities. Conventional or known methods formaking ceramic products with such shapes by compacting and sinteringparticles are not readily applicable, because the internal patternrequired to establish the desired part geometry cannot be easily removedafter the body is formed around it. While such part geometries cansometimes be prepared by machining or grinding the desired shape fromgreen preform or finished ceramic blank, this approach is undesirablebecause of the high costs of machining and grinding operations,especially when applied to ceramic materials. In many cases suchgeometries cannot presently be produced at all, even by machining orgrinding.

The invention described in Commonly Owned U.S. Pat. No. 4,828,785provides shape cavity-containing ceramic bodies typically of highstrength and fracture toughness by a mechanism which is more direct,more versatile and less expensive than conventional approaches. Theinvention described therein also provides means for reliably producingcavity-containing ceramic bodies of a size and thickness which isdifficult or impossible to duplicate with the presently availabletechnology. Briefly, the invention therein described involves embeddinga shaped parent metal precursor within a conformable filler, andinfiltrating the filler with a ceramic matrix obtained by oxidation ofthe parent metal to form a polycrystalline material consistingessentially of the oxidation reaction product of said parent metal withan oxidant and optionally, one or more metallic constituents. Moreparticularly, in practicing the invention, the parent metal is shaped toprovided a pattern, and then is surrounded by a conformable filler whichinversely replicates the geometry of the shaped parent metal. In thismethod, the filler (1) is permeable to the oxidant when required as inthe case where the oxidant is a vapor-phase oxidant and, in any case, ispermeable to infiltration by the developing oxidation reaction product;(2) has sufficient conformability over the heat-up temperature intervalto accommodate the differential thermal expansion between the filler andthe parent metal plus any melting-point volume change of the metal; and(3) at least in a support zone thereof enveloping the pattern, isintrinsically self-bonding, whereby said filler has sufficient cohesivestrength to retain the inversely replicated geometry within the bed uponmigration of the parent metal as described below. The surrounded shapedparent metal is heated to a temperature region above its melting pointbut below the melting point of the oxidation reaction product to form abody of molten parent metal. The molten parent metal is reacted in thattemperature region or interval with the oxidant to form the oxidationreaction product. At least a portion of the oxidation reaction productis maintained in that temperature region and in contact with and betweenthe body of molten metal and the oxidant, whereby molten metal isprogressively drawn from the body of molten metal through the oxidationreaction product, concurrently forming the cavity as oxidation reactionproduct continues to form within the bed of filler at the interfacebetween the oxidant and previously formed oxidation reaction product.This reaction is continued in the temperature region for a timesufficient to at least partially embed the filler within the oxidationreaction product by growth of the latter to form the composite bodyhaving the aforesaid cavity therein. Finally, the resultingself-supporting composite body is separated from excess filler, if any.

SUMMARY OF THE INVENTION

The present invention provides an alternative method for producingshaped, cavity-containing ceramic bodies. An expendable or disposablepattern is surrounded or embedded with a quantity of filler material.The pattern is then eliminated and replaced by a quantity of parentmetal, and the oxidation reaction proceeds with the resulting oxidationreaction product infiltrating the filler material as described above inconnection with the Commonly Owned Patent. The geometry of the cavityinversely replicates the geometry of the pattern.

In more detail, the method comprises shaping a disposable or expendablepattern of any suitable material such as plastic, foam, wax, or a lowmelting metal. The expendable pattern is packed within or surroundedwith a bed of conformable filler material to inversely replicate thegeometry of the expendable pattern in the bed. The pattern is theneliminated, as for example, by vaporization, and replaced with aquantity of parent metal, preferably molten. The bed and the body ofparent metal contained within it are then heated to a processtemperature above the melting point of the parent metal but below themelting point of the oxidation reaction product. In this temperatueinterval, the molten parent metal reacts with an oxidant, e.g., avapor-phase oxidant, to form the oxidation reaction product. At least aportion of the oxidation reaction product is maintained in contact withand between the body of molten metal and the oxidant, and molten metalis progressively drawn from the body of molten metal through theoxidation reaction product toward said bed of filler to concurrentlyform the cavity in said bed of filler as oxidation reaction productcontinues to form at the interface between the oxidant and previouslyformed oxidation reaction product. The reaction is continued for a timesufficient to at least partially infiltrate or embed the filler withinthe oxidation reaction product by growth of the latter to form acomposite body having a cavity therein. Where desired, the boundaries ofthe filler bed may be provided with a barrier means to substantiallyinhibit or prevent growth there beyond so as to facilitate achieving anet shape to the ceramic composite body. The resulting self-supportingcomposite body is separated from excess filler and/or parent metal, ifany.

The bed of filler material is characterized by being permeable to theoxidant when required as in the case when the oxidant is a vapor-phaseoxidant, and being permeable to infiltration by the developing oxidationreaction product. The expendable pattern, which is packed in the fillermaterial, may be removed as by vaporization, solution, melting anddraining, or the like, prior to adding the parent metal to the cavity.Metal is then added to the resulting cavity either as molten metal, oras a solid and then melted in place. In another embodiment, the moltenparent metal is poured onto the expendable pattern to vaporize thepattern. Where desired, the bed of filler material possesses sometemporary bonding strength to maintain the desired shape in the cavity.The oxidation reaction process is then conducted to form the composite.

Generally, it is relatively easy to shape an expendable patternmaterial. For example, expendable pattern materials such as expandedpolystyrene can be extruded, molded, or injection molded with relativeease, and therefore it is possible to produce by the present inventionceramic composites with cavities having a complicated or intricatecavity geometry or shape.

The product of this invention is a self-supporting ceramic compositebody having therein a cavity which inversely replicates the shape orgeometry of the expendable pattern and comprises a ceramic matrix havingfiller incorporated therein. The matrix consists essentially of apolycrystalline oxidation reaction product having interconnectedcrystallites formed upon oxidation of the parent metal precursor, andoptionally metallic constituents or pores or both.

The material of this invention can be grown with substantially uniformproperties through their cross section to a thickness heretoforedifficult to achieve by conventional processes for producing denseceramic structures. The process which yields these materials alsoobviates the high costs associated with conventional ceramic productionmethods, including fine, high purity, uniform powder preparation: greenbody forming: binder burnout: sintering: hot pressing and/or hotisostatic pressing. The products of the present invention are adaptableor fabricated for use as articles of commerce which, as used herein, isintended to include, without limitation, industrial, structural andtechnical ceramic bodies for such applications where electrical, wear,thermal, structural or other features or properties are important orbeneficial, and is not intended to include recycled or waste materialssuch as might be produced as unwanted by-products in the processing ofmolten metals.

As used in this specification and the appended claims, the terms beloware defined as follows:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body may contain minor orsubstantial amounts of one or more matallic constituents derived fromthe parent metal, or reduced from the oxidant or a dopant, mosttypically within a range of from about 1-40% by volume, but may includestill more metal.

"Oxidation reaction product" generally means one or more metals in anyoxidized state wherein a metal has given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of reaction of one or more metals with an oxidantsuch as those described in this application. "Oxidant" means one or moresuitable electron acceptors or electron sharers and may be a solid, aliquid, or a gas (vapor) or some combination of these (e.g., a solid anda gas) at the process conditions. "Parent material" refers to disposableor expendable materials such as plastics, foams, waxes, and low meltingmetals which can be extruded, molded, cast, machined, or otherwiseshaped for establishing the geometry of the cavity, and also which canbe chemically or physically removed from the bed of filler materialwhile leaving the cavity formed thereby substantially intact. "Parentmetal" as used in this specification and the appended claims refers tothe metal, e.g., aluminum, which is the precursor for thepolycrystalline oxidation reaction product, and includes that metal as arelatively pure metal, a commercially available metal with impuritiesand/or alloying constituents, or an alloy in which that metal precursoris the major constituent; and when a specified metal is mentioned as theparent metal, e.g., aluminum, the metal identified should be read withthis definition in mind unless indicated otherwise by the context."Cavity" means broadly an unfilled space within a mass or body, and isnot limited to any specific configuration of the space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, cross-sectional view in elevation showing anassembly of a pattern material surrounded by a bed of particulate fillerand confined within a refractory vessel;

FIG. 2 is a perspective view similar to FIG. 1 showing the addition of aparent metal to the cavity.

FIG. 3 is a cross-sectional view of a ceramic composite body of FIG. 1made in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

In the practice of the present invention, a quantity of a patternmaterial is provided in the form of an expendable pattern, the geometryof which is to be inversely replicated as a cavity within the finishedceramic composite. By following the practices of the present invention,complex shapes can be inversely replicated within the finished ceramiccomposite during formation or growth of the ceramic, rather than byshaping or machining a ceramic body. The term "inversely replicated"means that the cavity in the ceramic composite attained by the inventionprocess is defined by interior surfaces of the ceramic composite whichare congruent to the shape of the expendable pattern. The patternmaterial may be suitably shaped by any appropriate means; for example, aquantity of an expendable pattern material may be suitably molded,extruded, cast, machined or otherwise shaped. The pattern may havegrooves, bores, recesses, lands, bosses, flanges, studs, screw threadsand the like formed therein as well as having collars, bushings, discs,bars, or the like assembled thereto to provide patterns of virtually anydesired configuration. The pattern may be hollow or may comprise one ormore unitary pieces suitably shaped so that when surrounded within aconformable bed of filler, the pattern material defines a shaped cavitywithin the bed and occupies the cavity within the mass of filler.

When the expendable pattern material is eventually replaced by aquantity of parent metal which is melted under oxidation reactionconditions, a shaped cavity develops in the resulting ceramic compositebody. Thus, in one aspect, the present invention provides the advantageof making the cavity shape by molding, extruding, casting or machiningan expendable pattern material such as a plastic foam or a low meltingmetal rather than by forming, grinding or machining a ceramic, or byshaping the parent metal precursor as taught in the aforesaid CommonlyOwned U.S. Pat. No. 4,828,785.

The pattern materials which may be used in the present invention includethose materials which have been used in conventional expendable moldcasting techniques. Although various expendable grade waxes or waxblends are suitable for certain embodiments, expanded plastics, foams,and low melting metal alloys are preferred. More preferably,polystyrenes, polyethylenes, and polyurethanes are used as the patternmaterials.

The pattern material may be shaped by conventional processes includinginjection molding, blow molding, extrusion, casting, machining and thelike. Injection molding is currently preferred for making large numbersof patterns. Blow molding also may be preferred in other embodiments forits ability to produce hollow expendable molds. Blow molding may beparticularly desirable because it minimizes the amount of expendablematerial in order to facilitate a more rapid evacuation of the cavity.

The expendable material may be eliminated or evacuated from the cavityby various methods. In one embodiment, the expendable pattern materialmay be vaporized by evaporation or combustion prior to the addition ofthe parent metal precursor. In alternative embodiments, the patternmaterial may be removed by melting and allowing the material to drainfrom the cavity. Any residue can be burned out as in a prefiring step.The expendable pattern also may be dissolved by chemical means, and anyresidue washed from the cavity by use of a suitable solvent.

In still other alternative embodiments, the pattern material is left inplace until a quantity of molten parent metal is poured directly intothe cavity. When the molten parent metal contacts the pattern, thematerial is vaporized and thus eliminated from the cavity. In this way,molten parent metal concomitantly replaces the evacuating patternmaterial thereby reducing the chance of disturbing or upsetting the bedof filler. As a result, the filler material is more likely to retain theshape of the cavity.

Depending on the desired method of replacing the pattern material withthe parent metal, the parent metal may be added in either molten orsolid form, e.g., powder, particulate granules or pieces. The use of amolten parent metal is preferred because it completely fills the cavityat or near the temperature at which the oxidation reaction will occur.In addition, when the parent metal is in a molten state, a fresh surfaceof the parent metal is available for the oxidation reaction process,i.e. the surface is free of surface oxides, etc. Where desired, thefiller bed and expendable pattern may be placed in a furnace at or nearthe process temperature, and molten parent metal added to expel thepattern. In this manner, as molten metal is added and displaces thepattern which is being vaporized, the oxidation reaction begins andinfiltration of the bed occurs. In alternative embodiments, the patternis first displaced, and then the parent metal is poured into the cavity.A powedered or granulated parent metal may be desirable in someembodiments because interstices in the powdered or granulated mass as awhole would accommodate thermal expansion of the metal. Also, parentmetal in powdered or granulated form, when added to the cavity, wouldconform readily to the shape of the cavity in the bed of fillermaterial.

Although the invention is described below in detail with specificreference to aluminum as the preferred parent metal, other suitableparent metals which meet the criteria of the present invention include,but are not limited to, silicon, titanium, tin, zirconium and hafnium.

A solid, liquid or vapor-phase (gas) oxidant, or a combination of suchoxidants, may be employed. For example, typical oxidants include,without limitation, oxygen, nitrogen, a halogen, sulphur, phosphorus,arsenic, carbon, boron, selenium, tellurium, and compounds andcombinations thereof, for example, silica (as a source of oxygen),methane, ethane, propane, acetylene, ethylene, and propylene (as asource of carbon), and mixtures such as air, H₂ /H₂ O and CO/CO₂, thelatter two being useful in controlling the oxygen activity of theenvironment.

Although any suitable oxidants may be employed, specific embodiments ofthe invention are described below with reference to use of vapor-phaseoxidants. If a gas or vapor oxidant, i.e., a vapor-phase oxidant, isused the filler is permeable to the vapor-phase oxidant so that uponexposure of the bed of filler to the oxidant, the vapor-phase oxidantpermeates the bed of filler to contact the molten parent metal therein.The term "vapor-phase oxidant" means a vaporized or normally gaseousmaterial which provides an oxidizing atmosphere. For example, oxygen orgas mixtures containing oxygen (including air) are preferred vapor-phaseoxidants, as in the case where aluminum is the parent metal and aluminumoxide the desired reaction product, with air usually being morepreferred for obvious reasons of economy. When an oxidant is identifiedas containing or comprising a particular gas or vapor, this means anoxidant in which the identified gas or vapor is the sole, predominant orat least a significant oxidizer of the parent metal under the conditionsobtaining in the oxidizing environment utilized. For example, althoughthe major constituent of air is nitrogen, the oxygen content of air isthe normally sole oxidizer for the parent metal because oxygen is asignificantly stronger oxidant than nitrogen. Air therefore falls withinthe definition of an "oxygen-containing gas" oxidant but not within thedefinition of a "nitrogen-containing gas" oxidant. An example of a"nitrogen-containing gas" oxidant as used herein and in the claims is"forming gas", which contains 96 volume percent nitrogen and 4 volumepercent hydrogen.

When a solid oxidant is employed, it is usually dispersed through theentire bed of filler in the form of particulates admixed with thefiller, or perhaps as coatings on the filler particles. Any suitablesolid oxidant may be employed including elements, such as boron orcarbon, or reducible compounds, such as silicon dioxide or certainborides of lower thermodynamic stability than the boride reactionproduct of the parent metal. For example, when boron or a reducibleboride is used as a solid oxidant for an aluminum parent metal, theresulting oxidation reaction product is aluminum boride.

In some instances, the oxidation reaction may proceed so rapidly with asolid oxidant that the oxidation reaction product tends to fuse due tothe exothermic nature of the process. This occurrence can degrade themicrostructural uniformity of the ceramic body. This rapid exothermicreaction can be avoided by mixing into the composition relatively inertfillers which exhibit low reactivity. Such fillers absorb the heat ofreaction to minimize any thermal runaway effect. An example of such asuitable inert filler is one which is identical to the intendedoxidation reaction product.

If a liquid oxidant is employed, the entire bed of filler or a portionthereof adjacent the molten metal is coated or soaked as by immersion inthe oxidant to impregnate the filler. Reference to a liquid oxidantmeans one which is a liquid under the oxidation reaction conditions, andso a liquid oxidant may have a solid precursor, such as a salt, which ismolten at the oxidation reaction conditions. Alternatively, the liquidoxidant may have a liquid percursor, e.g., a solution of a material,which is used to impregnate part of all of the filler and which ismelted or decomposed at the oxidation reaction conditions to provide asuitable oxidant moiety. Examples of liquid oxidants as herein definedinclude low melting glasses.

The filler material utilized in the practice of the invention may be oneor more of a wide variety of materials suitable for the purpose. As usedherein and in the claims, when speaking of surrounding the expendablepattern with the filler material, it is intended to refer to packing orembedding the filler material around the expendable pattern, or layingthe filler material up against the expendable pattern. The fillermaterial should substantially conform to the geometry of the expendablepattern. For example, if the filler comprises particulate material suchas fine grains or powders of a refractory metal oxide, the pattern issurrounded by the filler so that the pattern defines a filled cavity(filled or occupied by the pattern). However, it is not necessary thatthe filler be in fine particulate form. For example, the filler maycomprise wire, fibers, hollow bodies, spheres, bubbles, pellets,platelets or aggregate, or whiskers, or such materials as metal wool,wires, or refractory fiber cloth. The filler also may comprise either aheterogeneous or homogeneous combination of two or more such componentsor geometric configurations, e.g., a combination of small particulategrains and whiskers. It is necessary only that the physicalconfiguration of the filler be such as to permit the expendable patternto be surrounded by or within a mass of the filler with the fillerclosely conforming to the surfaces of the pattern. The cavity ultimatelyformed in the ceramic composite is the negative of the geometry of thepattern material. This material initially forms a (filled) cavity withinthe bed of conformable filler, the cavity being initially shaped andfilled by the pattern material.

The filler material useful in the practice of the invention is onewhich, under the oxidation reaction conditions of the invention, ispermeable when the oxidant is a vapor-phase oxidant, to passagetherethrough of the oxidant. In any case, the filler also is permeableto the growth or development therethrough of oxidation reaction product.Where desired, the filler also has at the temperature at which theoxidation reaction is conducted, sufficient cohesive strength formed ordeveloped, so as to retain the geometry inversely replicated therein byconformance of the filler to the pattern material as the patternmaterial is replaced by the parent metal.

It is desirable to perform the method of the present invention in such away so as to minimize the time between the evacuation of the expendablepattern from the cavity and the point at which the reaction product hasformed in the filler material to produce a shell of sufficient strengthto maintain the shape of the cavity. However, there will be a transitionperiod, though brief, when the shape of the cavity is not maintained bythe pattern material or the reaction product. Thus, the filler materialdesirably possesses at least some capacity to self-bond so as tomaintain the shape of the cavity by the filler material alone.Otherwise, either the force of gravity on the filler or a pressuredifferential between the developing cavity and the process atmospherecould cause the cavity to collapse inwardly as it is evacuated by theparent metal.

One method of maintaining the geometry of the cavity is to use aself-bonding filler which, at the appropriate temperature, eitherintrinsically sinters and bonds or can be made to sinter or otherwisebond by appropriate additives or surface modifications of the filler.For example, a suitable filler for use with an aluminum parent metalutilizing an air oxidant comprises alumina powder with an added silicabonding agent as fine particles or coatings on the alumina powder. Suchmixtures of materials will partially sinter or bond at or below theoxidation reaction conditions under which the ceramic matrix will form.Without the silica additive, the alumina particles require substantiallyhigher temperatures for bonding.

Another suitable class of fillers includes particles or fibers which,under the oxidation reaction conditions of the process, form a reactionproduct skin on their surfaces which tends to bond the particles in thedesired temperature range. An example of this class of filler in thecase where aluminum is employed as the parent metal and air as theoxidant, is fine silicon carbide particles (e.g., 500 mesh and finer),which form a silicon dioxide skin bonding themselves together in theappropriate temperature range for the aluminum oxidation reaction.

In alternative embodiments, the geometry of the cavity can be maintainedduring the transition period by use of an organic binder material whichwill be evacuated from the filler material at or below the oxidationreaction temperature.

It is not necessary that the entire mass or bed of filler comprise asinterable or self-bonding filler or contain a sintering or bondingagent, although such arrangement is within the purview of the invention.The self-bonding filler and/or the bonding or sintering agent may bedispersed only in that portion of the bed or filler adjacent to andsurrounding the expendable pattern of parent metal to a depth sufficientto form upon sintering or otherwise bonding an encasement of thedeveloping cavity which is of sufficient thickness and mechanicalstrength to prevent collapse of the cavity (and consequent loss offidelity of its shape in the grown ceramic body to the shape of theexpendable pattern) before a sufficient thickness of the oxidationreaction product is attained. Thus, it suffices if a "support zone" offiller enveloping the pattern comprises a filler which is inherentlysinterable or self-bonding within the appropriate temperature range orcontains a sintering or bonding agent which is effective within theappropriate temperature range.

Moreover, the geometry of the cavity can be enhanced by utilizing as amandrel a low melting metal alloy. Low melting metals are desirable touse in isostatic pressing operations as well as sediment castingoperations. Particularly, when the metal is used as a mandrel in anisostatic pressing operation, the mandrel should have sufficientstrength to withstand the pressures of isostatic pressing of a fillermaterial thereabout. Moreover, the metal should be easily removable froma filler material which has been isostatically pressed thereabout byheating the alloy to a temperature of about 125°-300° C., thus resultingin the capability of forming complex cavity shapes. Removal at such lowtemperatures does not adversely effect the organic binder in the fillermaterial. Further, low melting alloys should exhibit minimal thermalexpansion or, preferably, negative thermal expansion, prior to meltingand thus will not stress the filler material, thereby ameliorating thetendency of the filler material to crack. Still further, the low meltingalloy should not wet the filler material to any significant extent,resulting in the filler material having a surface which is substantiallyfree from any residual low melting alloy. This enhances the inverseshape replication process by producing inversely replicated surfaceswhich are substantially free from defects. An example of a suitable lowmelting alloy is a low melting bismuth/tin alloy (e.g., approximately 58weight percent Bi and 42 weight percent Sn) known as CERROTUR andproduced by Cerro Metal Products of Bellefonte, Pa.

While the only low melting metal that has been discussed herein isCERROTUR, it is to be understood that other similar low melting alloys,which exhibit the properties discussed above herein, are acceptable foruse in the present invention.

As used herein and in the claims, a "support zone" of filler is thatthickness of filler enveloping the pattern which, upon bonding, is atleast sufficient to provide the structural strength necessary to retainthe replicated geometry of the expendable pattern material until thegrowing oxidation reaction product becomes self-supporting againstcavity collapse. The size of the support zone of filler will varydepending on the size and configuration of the pattern and themechanical strength attained by the sinterable or self-bonding filler inthe support zone. The support zone may extend from the surface of thepattern material into the filler bed for a distance less than that towhich the oxidation reaction product will grow or for the full distanceof growth. In fact, in some cases the support zone may be quite thin.For example, although the support zone of filler may be a bed of fillerencasing the pattern material and itself encased within a larger bed ofnon-self-bonding or non-sinterable filler, the support zone may insuitable cases comprise only a coating of self-bonding or sinterableparticles adhered to the expendable pattern by a suitable adhesive orcoating agent. An example of this coating technique is given below.

In any case, the filler should not sinter, fuse or react in such a wayas to form an impermeable mass so as to block the infiltration of theoxidation reaction product therethrough or, when a vapor-phase oxidantis used, passage of such vapor-phase oxidant therethrough. Any sinteredmass which does form should not form at such a low temperature as tofracture due to an expansion mismatch between the pattern material andthe filler before the vaporization temperature is reached.

As noted previously, a bonding or sintering agent may be included as acomponent of the filler in those cases where the filler would nototherwise have sufficient inherent self-bonding or sinteringcharacteristics to prevent collapse of the cavity being formed into thevolume formerly occupied by the expendable pattern. This bonding agentmay be dispersed throughout the filler or in the support zone only.Suitable materials for this purpose include organo-metallic materialswhich under the oxidizing conditions required to form the oxidationreaction product will at least partially decompose and bind the fillersufficiently to provide the requisite mechanical strength. The bindershould not interfere with the oxidation reaction process or leaveundesired residual by-products within the ceramic composite product.Binders suitable for this purpose are well known in the art. Forexample, tetraethylorthosilicate is exemplary of suitableorgano-metallic binders, leaving behind at the oxidation reactiontemperature a silica moiety which effectively binds the filler with therequisite cohesive strength.

It is presently preferred to pre-heat the bed of filler material beforethe parent metal is added thereto. In this way, thermal shock to the bedcan be avoided. It may be most effective to heat the bed of fillermaterial to the same or higher temperature of the molten parent metalwhich is poured into the cavity. After the pattern material has beenreplaced by the parent metal in the cavity, the set-up of the parentmetal and bed in an oxidizing environment is maintained at an oxidationreaction temperature above the melting point of the metal but below themelting point of the oxidation reaction product. As mentioned, theparent metal may be added to the cavity in the form of a powder,particles or pieces. In that event, the set-up is heated above themelting point of the metal thus producing a body or pool of moltenmetal.

On contact with the oxidant, the molten metal will react to form a layerof oxidation reaction product. Upon continued exposure to the oxidizingenvironment, within an appropriate temperature region, the remainingmolten metal is progressively drawn into and through the oxidationreaction product in the direction of the oxidant and into the bed offiller and, on contact with the oxidant, forms additional oxidationreaction product. At least a portion of the oxidation reaction productis maintained in contact with and between the molten parent metal andthe oxidant so as to cause continued growth of the polycrystallineoxidation reaction product in the bed of filler, thereby embeddingfiller within the polycrystalline oxidation reaction product. Thepolycrystalline matrix material continues to grow so long as suitableoxidation reaction conditions are maintained.

The process is continued until the oxidation reaction product hasinfiltrated or embedded the desired amount of filler. The resultingceramic composite product includes filler embedded by a ceramic matrixcomprising a polycrystalline oxidation reaction product and including,optionally, one or more non-oxidized constituents of the parent metal orvoids, or both. Typically in these polycrystalline ceramic matrices, theoxidation reaction product crystallites are interconnected in more thanone dimension, preferably in three dimensions, and the metal inclusionsor voids may be at least partially interconnected. When the process isnot conducted beyond the exhaustion of the parent metal, the ceramiccomposite obtained is dense and essentially void-free. When the processis taken to completion, that is, when as much of the metal as possibleunder the process conditions has been oxidized, pores in the place ofthe interconnected metal will have formed in the ceramic composite. Theresulting ceramic composite product of this invention possesses a cavityof substantially the original dimensions and geometric configuration ofthe original expendable pattern.

Referring now to the drawings, FIG. 1 shows a refractory vessel 2, suchas an alumina vessel, containing a bed of filler 4 which surrounds apattern, indicated generally by 6, of any suitable material such aspolystyrene. As shown in FIGS. 1 and 2, pattern 6 has a center section8, which is generally cylindrical in configuration, joined by an endsection 8a which is axially shorter but of greater diameter than centersection 8. In this embodiment, the filler is retained by a suitablebarrier means 10, such as a stainless steel screen or perforated steelcylinder which also establishes the boundaries of the ceramic body.Alternatively, the barrier may comprise a plaster of paris mold orcalcium silicate mold typically applied as a slurry to a substrate suchas cardboard and then allowed to set. The barrier thus defines theboundary or perimeter of the ceramic body by inhibiting growth of theoxidation reaction product therebeyond.

The pattern material 6, if foam, may be replaced by the parent metal bypouring molten parent metal 12 directly onto the pattern 6 in thecavity. In this way, the pattern material is vaporized and exits thecavity either through the bed of filler material, through the same portthrough which the parent metal was added, or through a separate ventingport (not shown) if the port through which the parent metal is added isrelatively small.

In an alternative embodiment, the expendable pattern is removed in astep prior to adding the molten parent metal. This may be accomplishedby melting the pattern and draining the melted material from the cavity,but also can be accomplished by placing the assembly in a furnace whichis heated to a point at which the expendable material is vaporized orburned. As mentioned above, the pattern material may also be removed byother techniques, such as dissolving the pattern, mechanically removingthe pattern, etc.

After the parent metal is added to the cavity, the assembly is heated toa temperature sufficient to melt the metal, if it was not added in amolten state. Thereafter, a sufficiently high temperature is maintainedwhereby a vapor-phase oxidant, which permeates the bed of filler 4, andis in contact with the molten metal, oxidizes the molten metal, andgrowth of the oxidation reaction product resulting therefrom infiltratesthe surrounding bed of filler 4.

For example, when the parent metal is an aluminum parent metal and airis the oxidant, the oxidation reaction temperature may be from about850° C. to about 1450° C., preferably from about 900° C. to about 1350°C., and the oxidation reaction product is typically alpha-alumina. Themolten metal migrates through the forming skin of oxidation reactionproduct from the volume formerly occupied by pattern material 6, therebyforming the composite with a cavity replicating the shape of thepattern.

In certain embodiments, it may be desirable to place a quantity of thefiller material over the port after the parent metal is added to thecavity. A closed cavity would thus be formed. In such embodiments, oreven in some cases without placing filler material over the port, themigration of the parent metal can result in a pressure drop within thatvolume, as in the case of a closed cavity, due to impermeability to thesurrounding atmosphere of the growing skin of oxidation reaction productin the bed of filler material and the skin of oxidation reaction productforming on top of the pool of molten metal. Thus a net external pressureacts on the container-like skin of oxidation reaction product. However,in a preferred embodiment the bed of filler 4 (or a support zonethereof) enveloping pattern 6 is intrinsically self-bonding at or abovea self-bonding temperature which preferably lies close to but below theoxidation reaction temperature. Thus, upon being heated to itsself-bonding temperature the filler, or a support zone thereof, hassintered or otherwise bonded to itself and attached to the growingoxidation reaction product sufficiently to afford adequate strength tothe filler surrounding the developing cavity, i.e., the support zone offiller, to resist the pressure differential and thereby retain withinthe bed of filler 4 the geometry of the cavity formed therein byconformance of the filler to the shape of pattern 6. Representing anembodiment in which only a support zone of filler 4 contains orcomprises a sinterable or self-bonding filler or a bonding or sinteringagent, dotted line 14 in FIG. 1 indicates the extent of the support zonein the bed of filler 4. As the reaction continues, the cavity within bed4 is partially or substantially entirely evacuated by the migration ofmolten parent metal through the oxidation reaction product to the outersurface thereof where it contacts the vapor-phase oxidant and isoxidized to form additional oxidation reaction product. The oxidationreaction product comprises a polycrystalline ceramic material which maycontain inclusions therein of unoxidized constituents of the moltenparent metal. Upon completion of the reaction, any remaining liquidmetal within the cavity may be eliminated by decanting it if growth of athick reaction product layer over the entry port has been prevented (asby using a barrier or inhibitor). Alternatively the assembly may beallowed to cool and any excess metal solidified and removed in asubsequent step such as acid leaching. The resultant ceramic composite,whose dimensions are indicated by the barrier 10, in FIG. 1, isseparated from excess filler, if any, left within vessel 2. Such excessfiller or part thereof may form a coherent mass or body because of thesintering or self-bonding, and this coherent mass may be removed fromthe ceramic composite which it encases by grit blasting, grinding, orthe like. An economical technique is to employ grit blasting utilizinggrit particles of a material which is suitable as the filler or as acomponent of the filler so that the removed filler and grit may bereused as filler in a subsequent operation. It is important to recognizethat the degree of strength of the self-bonded filler used to preventcavity collapse during processing is typically much less than thestrength of the resulting composite. Hence, it is in fact quite feasibleto remove excess self-bonded filler by rapid grit blasting withoutsignificant concern for damaging the resultant composite. In any case,the ceramic composite structure having the cavity formed therein may befurther shaped by machining or grinding or otherwise forming to adesired outer shape. In the example illustrated in FIG. 3, the ceramiccomposite 18 has the shape of a circular cylinder having an outersurface 20, end face 22 and cavity 24 which is defined by surfacescongruent to the surfaces of pattern 6. Thus, the shape of cavity 24 isan inverse replication of the shape of expendable pattern 6. For manyapplications, the ceramic body may be utilizable as formed followingremoval of the excess, unentrained filler, without further requirementfor grinding or machining.

By selecting an appropriate filler and maintaining the oxidationreaction conditions for a time sufficient to evacuate substantially allthe molten parent metal from the filled cavity initially occupied by thepattern material 6, a faithful inverse replication of the geometry ofpattern 6 is attained by cavity 16. While the illustrated shape ofpattern 6 (and therefore of cavity 16) is relatively simple, cavitiescan be formed within the ceramic composite which inversely replicatewith fidelity the shapes of much more complex geometry than that ofpattern 6 by the practices of the present invention. The outer surfacesof the ceramic composite may be shaped by placing a barrier means at thedesired locations to prevent growth therebeyond; in addition thesurfaces may be ground or machined or otherwise formed to any desiredsize or shape consistent with the size and shape of the cavity 16 formedtherein.

It should be understood that the filler properties of being permeable,conformable, and self-bonding (where desired) as described above areproperties of the overall composition of the filler, and that individualcomponents of the filler need not have any or all of thesecharacteristics. Thus, the filler may comprise either a single material,a mixture of particles of the same material but of different mesh size,or mixtures of two or more materials. In the latter case, somecomponents of the filler may, for example, not be sufficientlyself-bonding or sinterable at the oxidation reaction temperature but thefiller of which it is a component part will have the self-bonding orsintering characteristics at and above its self-bonding temperaturebecause of the presence of other materials. A large number of materialswhich make useful fillers in the ceramic composite by imparting desiredqualities to the composite also will have the permeable, conformable andself-bonding qualities described above. Such suitable materials willremain unsintered or unbonded sufficiently at temperatures below theoxidation reaction temperature so that the filler which surrounds thepattern can accommodate thermal expansion and any melting point volumechange of the pattern material and yet may sinter or otherwise self-bondonly upon attaining a self-bonding temperature which preferably liesclose to and below the oxidation reaction temperature, sufficiently toimpart the requisite mechanical strength to prevent collapse of theforming cavity during the initial stages of growth or development of theoxidation reaction product. Suitable fillers include, for example,silica, silicon carbide, alumina, zirconia, and combinations thereof.

As a further embodiment of the invention and as explained in theCommonly Owned Patents and Patent Applications, the addition of dopantmaterials to the metal can favorably influence the oxidation reactionprocess. The function or functions of the dopant can depend upon anumber of factors other than the dopant material itself. These factorsinclude, for example, the particular parent metal, the end productdesired, the particular combination of dopants when two or more dopantsare used, the concentration of the dopant, the oxidizing environment,and the process conditions.

The dopant or dopants may be provided as alloying constituents of theparent metal or may be applied to the filler or to a part of the fillerbed, e.g., the support zone of the filler, or both. In the case of thesecond technique, where a dopant or dopants are applied to the filler,the application may be accomplished in any suitable manner, such as bydispersing the dopants throughout part or the entire mass of filler ascoatings or in particulate form, preferably including at least a portionof the bed of filler adjacent the parent metal. Application of any ofthe dopants to the filler may also be accomplished by applying a layerof one or more dopant materials to and within the bed, including any ofits internal openings, interstices, passageways, intervening spaces, orthe like, that render it permeable. A convenient manner of applying anyof the dopant material is to merely soak the entire bed in a liquidsource (e.g., a solution) of dopant material. A source of the dopant mayalso be provided by placing a rigid body of dopant in contact with andbetween at least a portion of the expendable pattern surface and thefiller bed. For example, a thin sheet of silica-containing glass (usefulas a dopant for the oxidation of an aluminum parent metal) can be placedupon a surface of the expendable pattern. When the expendable pattern isreplaced by a quantity of molten aluminum parent metal (which may alsobe internally doped) and the resulting assemblage is heated in anoxidizing environment (e.g. in the case of aluminum in air, betweenabout 850° C. to about 1450° C., or preferably about 900° C. to about1350° C.), growth of the polycrystalline ceramic material into thepermeable bed occurs. In the case where the dopant lies between theparent metal and the bed of filler material, the polycrystalline oxidestructure generally grows within the permeable filler substantiallybeyond the dopant layer (i.e., to beyond the depth of the applied dopantlayer). In any case, one or more of the dopants may be externallyapplied to the expendable pattern surface and/or to the permeable bed.Additionally, dopants alloyed within the parent metal may be augmentedby dopant(s) applied to the filler bed. Thus, any concentrationdeficiencies of the dopants alloyed within the parent metal may beaugmented by an additional concentration of the respective dopant(s)applied to the bed, and vice versa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium and zinc, especially incombination with other dopants as described below. These metals, or asuitable source of the metals, may be alloyed into the aluminum-basedparent metal at concentrations for each of between about 0.1-10% byweight based on the total weight of the resulting doped metal. Theconcentration for any one dopant will depend on such factors as thecombination of dopants and the process temperature. Concentrationswithin the appropriate range appear to initiate the ceramic growth,enhance metal transport and favorably influence the growth morphology ofthe resulting oxidation reaction product.

Other dopants which are effective in promoting polycrystalline oxidationreaction product growth, especially for aluminum-based parent metalsystems are, for example, silicon, germanium, tin and lead, especiallywhen used in combination with magnesium or zinc. One or more of theseother dopants, or a suitable source of them, is alloyed into thealuminum parent metal system at concentrations for each of from about0.5 to about 15% by weight of the total alloy; however, more desirablegrowth kinetics and growth morphology are obtained with dopantconcentrations in the range of from about 1-10% by weight of the totalparent metal alloy. Lead as a dopant is generally alloyed into thealuminum-based parent metal at a temperature of at least 1000° C. so asto make allowances for its low solubility in aluminum; however, theaddition of other alloying components, such as tin, will generallyincrease the solubility of lead and allow the alloying material to beadded at a lower temperature.

Additional examples of dopant materials useful with an aluminum parentmetal include sodium, lithium, calcium, boron, phosphorous and yttriumwhich may be used individually or in combination with one or moredopants depending on the oxidant and process conditions. Sodium andlithium may be used in very small amounts in the parts per millionrange, typically about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful dopants, and herein again especially when used incombination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all, or a portion of, the surface ofthe expendable pattern enables local ceramic growth from the parentmetal or portions thereof and lends itself to growth of thepolycrystalline ceramic material into the permeable filler in selectedareas. Thus, growth of the polycrystalline ceramic material into thepermeable bed can be controlled by the localized placement of the dopantmaterial upon the surface of the expendable pattern. The applied coatingor layer of dopant is thin relative to the intended thickness of ceramiccomposite, and growth or formation of the oxidation reaction productinto the permeable bed extends to substantially beyond the dopant layer,i.e., to beyond the depth of the applied dopant layer. Such layer ofdopant material may be applied by painting, dipping, silk screening,evaporating, or otherwise applying the dopant material in liquid orpaste form, or by sputtering, or by simply depositing a layer of a solidparticulate dopant or a solid thin sheet or film of dopant onto thesurface of the expendable pattern. The dopant material may, but neednot, include either organic or inorganic binders, vehicles, solvents,and/or thickeners. More preferably, the dopant materials are applied aspowders to the surface of the expendable pattern or dispersed through atleast a portion of the filler. One particularly preferred method ofapplying the dopants to the parent metal surface is to utilize a liquidsuspension of the dopants in a water/organic binder mixture sprayed ontoan expendable pattern surface in order to obtain an adherent coatingwhich facilitates handling of the expendable pattern prior toprocessing.

The dopant materials when used externally are usually applied to atleast a portion of a surface of the expendable pattern metal as auniform coating thereon. The quantity of dopant is effective over a widerange relative to the amount of parent metal to be reacted, and, in thecase of aluminum, experiments have failed to identify either upper orlower operable limits. For example, when utilizing silicon in the formof silicon dioxide externally applied as a dopant for analuminum-magnesium parent metal using air or oxygen as the oxidant,quantities as low as 0.00003 gram of silicon per gram of parent metal,or about 0.0001 gram of silicon per square centimeter of parent metalsurface on which the SiO₂ dopant is applied, are effective. It also hasbeen found that a ceramic structure is achievable from analuminum-silicon parent metal using air or oxygen as the oxidant byusing MgO as a dopant in an amount greater than about 0.0008 gram of Mgper gram of parent metal to be oxidized and greater than about 0.003gram of Mg per square centimeter of parent metal surface upon which theMgO is applied.

A barrier means may be used in conjunction with the filler material toinhibit growth or development of the oxidation reaction product beyondthe barrier, especially when vapor-phase oxidants are employed in theformation of the ceramic body. Suitable barrier means may be anymaterial, compound, element, composition, or the like, which, under theprocess conditions of this invention, maintains some integrity, is notvolatile, and preferably is permeable to the vapor-phase oxidant whilebeing capable of locally inhibiting, poisoning, stopping, interferingwith, preventing, or the like, continued growth of oxidation reactionproduct. Suitable barriers for use with aluminum parent metal includecalcium sulfate (Plaster of Paris), calcium silicate, and Portlandcement, and mixtures thereof, which typically are applied as a slurry orpaste to the surface of the filler material. These barrier means alsomay include a suitable combustible or volatile material that iseliminated on heating, or a material which decomposes on heating, inorder to increase the porosity and permeability of the barrier means.Still further, the barrier means may include a suitable refractoryparticulate to reduce any possible shrinkage or cracking which otherwisemay occur during the process. Such a particulate having substantiallythe same coefficient of expansion as that of the filler bed isespecially desirable. For example, if the preform comprises alumina andthe resulting ceramic comprises alumina, the barrier may be admixed withalumina particulate, desirably having a mesh size of about 20-1000, butmay be still finer. Other suitable barriers include a stainless steelscreen, refractory ceramics or metal sheaths which are open on at leastone end or the walls perforated to permit a vapor-phase oxidant (ifused) to permeate the bed and contact the molten parent metal.

The ceramic composite structures obtained by the practice of the presentinvention will usually be a relatively dense, coherent mass whereinbetween about 5% and about 98% by volume of the total volume of thecomposite structure is comprised of one or more of the filler componentswhich are embedded within a polycrystalline ceramic matrix. Thepolycrystalline ceramic matrix is usually comprised of, when the parentmetal is aluminum and air or oxygen is the oxidant, about 60% to about99% by weight (of the weight of polycrystalline matrix) ofinterconnected alpha-alumina and about 1% to 40% by weight (same basis)of non-oxidized metallic constituents, such as from the parent metal.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

A styrofoam cup, about 7.5 cm long and having a base diameter of about4.5 cm and a wall thickness of 0.3 cm, was coated with a mixture of 95%silica and 5% clay by applying a water slurry of the silica and clay tothe cup (just short of the open end thereof) and heatig to dryness. Thecoating thickness was about the same as the wall thickness of the cup.The coated cup was buried in a bed of loose wollastonite with the end ofthe coating essentially flush with the exposed surface of the bed.

The cup was filled with molten 380.1 aluminum alloy (vaporizing thestyrofoam) and the matal/bed assembly placed in a hot furnace where itwas heated at 1000° C. for 48 hours.

The resulting ceramic body was removed from the wollastonite bed, theresidual molten aluminum alloy decanted, and the product allowed tocool, leaving a ceramic cup having an internal surface which replicatedin detail the external surface of the styrofoam cup. The externalsurface of the ceramic was defined by the wollastonite barriersurrounding the original coated pattern. The wall of the ceramic cup wascomprised of an alumina ceramic which had grown through the thickness ofthe silica/clay coating.

EXAMPLE 2

The procedure described in Example 1 was repeated with the exceptionthat alumina particles (Norton 38 Alundum of 70% 220 and 30% 500 meshparticle size) was substituted for the wollastonite, and the assemblywas heated for 72 hours. In this case, the alumina matrix grew throughthe thickness of the silica/clay coating and, into the surroundingalumina particles, forming a wall measuring up to about 0.6 cm. Againthe internal surface of the ceramic composite replicated the externalsurface of the styrofoam cup pattern.

Although only a few exemplary embodiments of the invention have beendescribed in detail above, those skilled in the art will readilyappreciate that the present invention embraces many combinations andvariation other than those exemplified.

What is claimed is:
 1. A method for producing a self-supporting ceramiccomposite body having therein at least one cavity which inverselyreplicates the geometry of at least one pattern, said composite bodycomprising a ceramic matrix obtained by oxidation of a parent metal toform a polycrystalline material comprising an oxidation reaction productof said parent metal with an oxidant, and a filler infiltrated by saidceramic matrix, the method comprising the steps of:(a) providing atleast one expendable pattern material comprising a low melting metal,said low melting metal comprising a metal which does not wet the fillermaterial; (b) packing said at least one expendable pattern materialwithin a bed of conformable filler to inversely replicate therein thegeometry of said at least one pattern material, said bed of filler beingcharacterized by (1) being permeable to said oxidant under the processconditions in step (c) and being permeable to infiltration by the growthof the oxidation reaction product through said filler, and (2) at leastin a support zone thereof enveloping said at least one pattern materialhaving sufficient cohesive strength under the process conditions in step(c) to retain the inversely replicated geometry within said bed; (c)heating said at least one pattern material to a temperature above itsmelting point to remove it from said bed of conformable filler; (d)replacing said at least one pattern material with a quantity of parentmetal and maintaining a temperature above the melting point of saidparent metal but below the melting point of said oxidation reactionproduct to maintain a body of molten parent metal and, at saidtemperature,(1) reacting the molten parent metal with said oxidant toform said oxidation reaction product, (2) maintaining at least a portionof said oxidation reaction product in contact with and between said bodyof molten metal and said oxidant, to progressively transport moltenmetal from said body of molten metal through the oxidation reactionproduct and into said bed of filler to concurrently form said at leastone cavity in said bed of filler as fresh oxidation reaction productcontinues to form at the interface between said oxidant and previouslyformed oxidation reaction product, and (3) continuing said reacting fora time sufficient to at least partially infiltrate said filler with saidoxidation reaction product by growth of the latter to form saidself-supporting ceramic composite body having said at least one cavitytherein; and (e) recovering the resulting self-supporting ceramiccomposite body.
 2. The method of claim 1, wherein said bed ofconformable filler material is isostatically pressed about said at leastone pattern material.
 3. The method of claim 1, wherein said bed ofconformable filler material is sedimentation cast about said at leastone pattern material.
 4. The method of claim 1, wherein said quantity ofparent metal added is molten.
 5. The method of claim 1, wherein saidoxidant comprises a vapor-phase oxidant.
 6. The method of claim 1,wherein said parent metal comprises an aluminum parent metal.
 7. Themethod of claim 1, wherein said oxidant comprises at least one oxidantselected from the group consisting of a vapor-phase oxidant, a solidoxidant and a liquid oxidant.
 8. The method of claim 1, wherein saidoxidant comprises a material selected from the group consisting ofsilica, boron, and a compound reducible by the parent metal.
 9. Themethod of claim 1, wherein said oxidant comprises at least one materialselected from the group consisting of an oxygen-containing gas and anitrogen-containing gas.
 10. The method of claim 1, wherein said parentmetal comprises at least one metal selected from the group consisting ofaluminum, silicon, titanium, tin, zirconium, and hafnium.
 11. The methodof claim 1, wherein said oxidant comprises at least one materialselected from the group consisting of an oxygen-containing gas,nitrogen-containing gas, a halogen, sulphur, phosphorus, arsenic,carbon, boron, selenium, tellurium, and compounds and mixtures thereof,methane, ethane, propane, acetylene, ethylene, propylene, a CO/CO₂mixture and a H₂ /H₂ O mixture, and mixtures thereof.
 12. The method ofclaim 1, wherein said filler material comprises at least one materialselected from the group consisting of hollow bodies, particulates,powder, fibers, whiskers, spheres, bubbles, steel wool, aggregate,wires, platelets, pellets, refractory fiber cloth and mixtures thereof.13. The method of claim 1, further comprising using at least one dopantmaterial in conjunction with said parent metal.
 14. The method of claim1, further comprising providing at least one dopant material at leastpartially within said filler material.
 15. The method of claim 14,wherein said at least one dopant material comprises at least twomaterials selected from the group consisting of magnesium, zinc,silicon, germanium, tin, lead, boron, sodium, lithium, calcium,phosphorus, yttrium, and rare earth metals.
 16. The method of claim 15,wherein said rare earth metal comprises at least one material selectedfrom a group consisting of lanthanum, cerium, praseodymium, neodymiumand samarium.
 17. The method of claim 1, further comprising a bondingagent incorporated into said filler material, at least in said supportzone thereof.
 18. The method of claim 13, wherein said parent metalcomprises aluminum, said dopant comprises a source of magnesium andsilicon, and said oxidant comprises air.
 19. The method of claim 1,wherein said low-melting metal comprises a metal which melts at atemperature of about 125°-300° C.
 20. The method of claim 1, whereinsaid low-melting metal comprises a material which has a negative thermalexpansion prior to melting.
 21. The method of claim 1, wherein saidlow-melting metal comprises a bismuth-tin alloy.
 22. The method of claim21, wherein a weight percent of bismuth is about 58 and a weight percentof tin is about 42.